The main objective of this research is to purify the ribonucleotide reductase from Ehrlich tumor cells so that the properties of this enzyme can be unequivocally established. It is felt that defining the properties and requirements of this enzyme, which catalyzes the formation of deoxyribonucleotides, will lead to a rational approach for the design of specific inhibitors of this key metabolic site. These inhibitors should be useful in the chemotherapy of neoplasms. The ribonucleotide reductase from Ehrlich tumor cells has been separated into two nonidentical components, neither of which has enzyme activity. However, the enzyme activity is reconstituted when both components are mixed. Each component has been extensively purified. It has been determined in both tumor cells and regenerating liver that the effector-binding subunit is the limiting component of the ribonucleotide system. The half-life of residual reductase activity is a composite of the half-lives of the individual subunits making up the active species. Significant differences in the properties of the individual subunit proteins exist such as heat-sensitivity and susceptibility to trypsin and chymotrypsin, which would account for different rates of turnover of the individual subunits. This information will be the basis of further studies directed at understanding the regulation of DNA replication through the ribonucleotide reductase site. (B)